Our next rental is going to get a glass top range (the one in our house currently) and we're going back to one with coils.

As for the unit freezing up; I am a HVAC contractor and if the unit is installed with a blower size that matches the evaporator coil the unit will never freeze up.

If you are just swapping out a new FAU with furnace, condensor and evap coil and not touching any ductwork and don't have to replace line set and can use the existing electrical, you are looking at about $4500 or so for a 4ton unit.

Assuming it's steam with a domestic coil.

Mine was a heater coil with so much dirt and pet hair it looked like a shag carpet was draped over it. 

Do I truly need to replace the whole system, given that the inside coil and blower fan, in my view, doesn’t look too bad? 

Clean filters inside and a clean coil outside guarantee this.

On a building that size look into vertical stack fan coil units also. 

HVAC System:The HVAC systems consists of 3 components – 2 furnaces (manufactured in 1994 & 1994), 2 AC evaporator coils (manufactured in 2015 & estimated in 2001) and 2 AC condenser fans (manufactured in 1995 & 2001).The 1st floor furnace is not operational and 2nd floor AC is not operational.

No idea what they vaped but it did leave a residue.Then again I had this new evap coil look like this after 6 months from smokers.